The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the recovery of signal drop out in MR images caused by local susceptibility gradients at tissue boundaries.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is well known that imperfections in the linear magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) produce artifacts in the reconstructed images. It is a well known problem, for example, that eddy currents produced by gradient pulses will distort the magnetic field and produce image artifacts. Methods for compensating for such eddy current errors are also well known as disclosed, for example, in U.S. Pat. Nos. 4,698,591; 4,950,994; and 5,226,418. It is also well known that the gradients may not be perfectly uniform over the entire imaging volume, which may lead to image distortion. Methods for compensating this non-uniformity are well known, and for example, are described in U.S. Pat. No. 4,591,789.
MR imaging methods are also dependant on the presence of a homogeneous polarizing magnetic field B.sub.0. Many methods have been devised to obtain a homogeneous and stable polarizing magnetic field, including the use of shim coils which can be periodically adjusted during calibration procedures. Such measures cannot, however, correct for magnetic field inhomogeneities arising from local susceptibility gradients produced when a patient is placed in the magnetic field. Such susceptibility gradients arise near air-tissue boundaries, for example, and in mild cases this can lead to reduced signal intensities in pixels located near such boundaries, and in severe cases it can lead to image artifacts such as pixel shifts and complete signal loss.
Methods have been proposed to recover the signal loss and signal dropout in MR images due to such susceptibility gradient inhomogeneities. These include using 3D-gradient echo methods, using higher image resolution, using tailored rf excitation pulses, and using multi-gradient echo acquisitions with susceptibility inhomogeneity compensation. Recognizing that the signal loss or drop out is due to the dephasing of the transverse magnetization in the local regions containing susceptibility gradients, another prior approach is to acquire multiple images and vary the slice refocusing gradient from the nominal value dictated by NMR theory. The multiple images may be summed together, or the physician may examine each image and combine the results in his own mind.